1. Field of the Invention
The present invention relates generally to an inkjet printer system and, more particularly to apparatus and methods for detecting a missing nozzle in the printhead of an inkjet printer.
2. Description of the Related Art
Inkjet printers employ a printhead having a plurality of nozzles for ejecting a microdroplet of ink onto a print media, such as paper. In many printers the printhead is moved laterally back and forth in a swath and the paper is scrolled, so that the desired text or image is printed on the print media. Other printing techniques can utilize a stationary printhead and a carriage mechanism that moves the paper both laterally and vertically. The printhead is constructed using a semiconductor structure with numerous holes or nozzles formed therein, which are connected to an ink delivery channel. Many printers have a number of arrays of nozzles, one array for printing cyan, one for yellow, one for magenta and one for black. Some printers also include a redundant array of nozzles. A heater formed in the semiconductor structure can be energized to heat the ink adjacent the nozzle to nucleate the ink into a droplet that is ejected forwardly from the nozzle opening. Generally, nozzle diameters range from about 5 to 20 microns. In view of the very small nozzle opening, a single microdroplet of ink can be difficult to see with the naked eye. Because of the very small size of the printhead nozzles, they can be clogged or otherwise prevented from operating properly. Ink or air can clog the nozzles, the ink heater for a nozzle can become defective, and many other printhead malfunctions can occur to prevent the proper ejection of ink from a nozzle.
During the normal operation of an inkjet printer, the controller is programmed to periodically perform a maintenance routine to simultaneous activate all nozzles numerous times to eject ink therefrom. The printhead maintenance routine is often carried out by moving the printhead to an extreme left or right carriage position where the nozzles are directed to a “spit cup” or container. The spit cup contains the dispensed ink therein. When in the maintenance position, the controller proceeds through the routine in which all nozzles are addressed plural times to simultaneously eject ink in an attempt to clean the same and provide reliable operation. This procedure can be carried out prior to the printing of a print job, after the printer has been inactive for a certain period of time, or for other reasons.
With some inkjet printers, defective nozzles can be detected by printing a sample after the printhead maintenance has been completed. An array of detector diodes is provided to sense the dot pattern on the printed sample. If the test shows that all of the dots are present, then it is assumed that all of the nozzles are operating properly. The disadvantage of this printhead test is that paper is used and additional time is required.
If it is determined that one or more nozzles are inoperative, then other corrective measures can be employed. For example, the controller can automatically carry out programmed routines to use neighbor nozzles and move the paper or printhead accordingly in order to compensate for the inoperative nozzle, all without significantly compromising the quality of the print job. If a number of nozzles are inoperative, then the time to print the job may increase due to the use of the extra compensating measures.
In view of the foregoing, it can be seen that a need exists for a technique to quickly test the printhead to determine if any nozzle is defective, and the particular nozzle that is defective. During the printhead cleaning operation, it would be advantageous to also determine whether any of the nozzles are defective or “missing,” without printing a sample.